Glossary
Created By Operations Training Solutions (O-T-S)
June 25, 2015
Glossary 10/7/15
A-Index
A daily index of geomagnetic activity for a specific station or network of stations derived as the average of the 8, 3-hourly K indices. The A-Index varies from 0 to 400.
AP-Index
A 3-hour equivalent amplitude of magnetic activity based on KP-Index data from a planetary network of magnetic observatories. NOAA’s SWPC estimates the value of the AP index by measuring the geomagnetic field in near real time at magnetometer stations around the world. The real-time station indices are used to estimate the official KP & AP based on historical data.
Accelerating Power
In a generator, the difference between the mechanical input power & the electrical output power (assuming no losses). For a generator’s torque angle to change, the generator must have accelerating power.
Accumulated Inadvertent
The accumulation over time of inadvertent energy. The accumulated inadvertent account of a BA represents the amount of excess energy a BA has either supplied to the interconnection or absorbed from the interconnection. BAs monitor their accumulated inadvertent to ensure the accounts do not grow too large.
Active Power (MW)
The component of the complex power (MVA) that performs the work. The common unit of active power is the MW & the symbol is “P”. Active power is sometimes called real power.
Active Power Transmission Losses
The power consumed due to resistive characteristics of transmission elements. Calculated as I2R.
Actual Net Interchange (NIA)
Algebraic sum of actual MW transfers across all Tie Lines & includes pseudo-ties. BAs directly connected via asynchronous ties to another Interconnection may include or exclude MW transfers on those tie lines in their actual interchange, provided they are implemented in the same manner for net interchange schedule.
Actual Load
The actual MW drawn by a load from the power system. The actual load will be different from the rated or nominal load if the load’s voltage or frequency vary from nominal values.
Adequacy
The ability of the electric system to supply the aggregate electrical demand and energy requirements of the end-use customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements. (NERC Glossary of Terms)
Adequate Regulating Margin
The minimum on-line capacity that can be increased or decreased to allow the system to respond to all reasonable demand changes in order to be incompliance with the CPS.
Adjacent Balancing Authority
A Balancing Authority whose Balancing Authority Area is interconnected with another Balancing Authority Area either directly or via a multi-party agreement or transmission tariff. (NERC Glossary of Terms)
Adverse Reliability Impact
The impact of an event that results in Bulk Electric System instability or Cascading. (NERC Glossary of Terms)
After the Fact (ATF)
A time classification assigned to an RFI when the submittal time is greater than one hour after the start time of the RFI. (NERC Glossary of Terms)
AGC Pulses
The AGC system will send signals to selected (regulating) generators to adjust their set-points. These signals are sent via telecommunication equipment. The signals are often called AGC pulses.
Aggregator
An entity responsible for planning, scheduling, accounting, billing, & settlement for energy deliveries from the aggregator’s choice of sellers &/or buyers. Aggregators seek to bring together customers or generators so they can buy or sell power in bulk, making a profit on the transaction.
Air-Gap
The air space between the rotor & stator of a motor or generator.
Alternating Current (AC)
A time varying electric current that changes periodically in magnitude & direction. In power systems, the changes follow the pattern of a sine wave having a frequency of 60 cycles per second in North America. AC is also used to refer to voltage, which follows a similar sine wave pattern.
Alternative Interpersonal Communication
Any Interpersonal Communication that is able to serve as a substitute for, and does not utilize the same infrastructure (medium) as, Interpersonal Communication used for day-today operation. (NERC Glossary of Terms)
Alternator
A rotating machine that develops alternating voltage & current outputs.
Aluminum Cable Steel Reinforced (ACSR)
A common type of transmission line conductor that comes in many different sizes & designs. The outer aluminum strands form the conductor & while the inner steel strands provide strength.
Ambient Conditions
Common, prevailing, & uncontrolled atmospheric conditions at a particular location, either indoors or out. The term is often used to describe the temperature, humidity, & airflow or wind that equipment or systems are exposed to.
Amortisseur Winding
A machine winding consisting of a number of conducting bars attached between the magnetic poles of the machine’s rotor. Amortisseur windings may be used as starter windings or to help dampen power oscillations.
Ampere (A)
The unit of measurement of electrical current produced in a circuit by 1 volt acting through a resistance of 1 Ohm.
Amplitude
The value or magnitude that a waveform has at a specific point in time.
Analog Electronics
Electronic circuitry in which the magnitudes of quantities are used in a continuous manner to perform functions. For example, input current values may be amplified & used to perform work in an analog circuit. Analog is different than digital in the sense that analog circuits use voltage & current magnitudes continuously while digital circuits use voltage & current to determine the state (on or off) of elements.
Ancillary Service
Those services that are necessary to support the transmission of capacity and energy from resources to loads while maintaining reliable operation of the Transmission Service Provider's transmission system in accordance with good utility practice. (From FERC order 888-A.) (NERC Glossary of Terms)
Angle Instability
The opposite of angle stability. When a power system loses angle stability it enters a period of angle instability. An angle unstable system has lost the magnetic bound that holds sections of an interconnected AC power system in synchronism with 1 another.
Angle Stability
An angle stable power system is 1 in which all elements of the power system are bound together via magnetic forces. For example, an angle stable generator’s internal magnetic field rotates in synchronism (in-step) with the magnetic field of the 3F power system to which it connects.
Anti-Aliasing Filter
An analog filter installed at a metering point to remove the high frequency components of the signal over the AGC sample period. (NERC Glossary of Terms)
Apparent Power
The product of the voltage & current phasors. Apparent power comprises both active & reactive power, usually expressed in kVA or MVA. Another term for apparent power is complex power.
Arc
The discharge of current through the air or in a gas.
Arcing Horns
An enhancement to a disconnect switch to increase the switch’s current interrupting capability.
Area Control Error (ACE)
Means the instantaneous difference between net actual and scheduled interchange, taking into account the effects of Frequency Bias including correction for meter error. (NERC Glossary of Terms)
Area Interchange Methodology
The Area Interchange methodology is characterized by determination of incremental transfer capability via simulation, from which Total Transfer Capability (TTC) can be mathematically derived. Capacity Benefit Margin, Transmission Reliability Margin, and Existing Transmission Commitments are subtracted from the TTC, and Postbacks and counterflows are added, to derive Available Transfer Capability. Under the Area Interchange Methodology, TTC results are generally reported on an area to area basis. (NERC Glossary of Terms)
Armature
The component of an electrical machine in which the voltage is induced. In a synchronous generator the armature is usually the stator. The armature winding is wrapped about the armature.
Arranged Interchange
The state where a Request for Interchange (initial or revised) has been submitted for approval. (NERC Glossary of Terms)
Arrested Frequency
Point of maximum frequency excursion in the 1ST swing of the frequency excursion between time zero (point “A”) & time zero plus 20 seconds. Also called the frequency nadir.
Arresting Period Frequency Response
Combination of load damping & the initial primary control response acting together to limit the duration & magnitude of frequency change during the arresting period.
Asynchronous
To be out-of-step with a reference. An induction generator is an asynchronous generator as its rotor does not rotate in synchronism with the power system.
Attaining Balancing Authority
A Balancing Authority bringing generation or load into its effective control boundaries through a Dynamic Transfer from the Native Balancing Authority. (NERC Glossary of Terms)
Aurora
Phenomenon associated with geomagnetic activity that is visible in the night sky. Aurorae occur within a band of latitudes known as the auroral oval, the location of which is dependent on geomagnetic activity. Aurorae are a result of collisions between atmospheric gases & charged particles (mostly electrons) guided by the geomagnetic field from the Earth’s magnetotail. Each gas (oxygen, nitrogen, etc.) emits a particular color depending on the energy of the particles & atmospheric composition. Auroral altitude range is 80 to 1,000 kM. Aurorae in the Northern Hemisphere are called Aurora Borealis while aurorae in the Southern Hemisphere are called Aurora Australis.
Aurora Oval
Elliptical band around each geo-magnetic pole ranging from about 75° magnetic latitude at local noon to about 67° at midnight under average conditions. These locations experience the maximum occurrence of aurorae. The aurora widens to both higher & lower latitudes during magnetic storms.
Auto-Load
A feature on some generators that automatically moves the generator to a target load (MW) level.
Automatic Generation Control (AGC)
Equipment that automatically adjusts generation in a Balancing Authority Area from a central location to maintain the Balancing Authority’s interchange schedule plus Frequency Bias. AGC may also accommodate automatic inadvertent payback and time error correction. (NERC Glossary of Terms)
Automatic Operating Systems
SPS, RAS, or other operating systems installed on electric systems that require no intervention on the part of system operators.
Automatic Reclosing
Device that automatically recloses a CB after it has been tripped by a protective relay. Automatic reclosing is applied on both distribution & transmission overhead lines. Since many faults clear automatically, automatic reclosing improves service continuity. Automatic reclosing applied in less than 1 second is called high speed automatic reclosing. NERC includes both the reclosing device and its associated control circuitry in its definition of automatic reclosing.
Automatic Time Error Correction
A frequency control automatic action that a BA uses to offset its frequency contribution to support the Interconnection’s scheduled frequency.
Auto Transformer
A power transformer with a single coil for each electrical phase, as opposed to a conventional transformer, which has 2 coils per phase. In an auto-transformer, the entire coil acts as the primary winding while a portion of the same coil acts as the secondary winding.
Auxiliary Relay
A relay whose function is to supplement the actions of other types of relays. For example, a lockout relay is an auxiliary relay with numerous contacts that each can perform an action. ANSI has numbers assigned to 2 types of auxiliary relays; 86 (lock-out) & 94 (tripping).
Availability
A measure of time a generating unit, transmission line, or other facility is capable of providing service, whether or not it actually is in service. Typically, this measure is expressed as a percent available for the period under consideration.
Available Resource
The sum of existing generating capacity, plus new units scheduled for service, plus the net of equivalent firm capacity purchases & sales, less existing capacity unavailable due to planned outages.
Available Flowgate Capability (AFC)
A measure of the flow capability remaining on a Flowgate for further commercial activity over and above already committed uses. It is defined as TFC less Existing Transmission Commitments (ETC), less a Capacity Benefit Margin, less a Transmission Reliability Margin, plus Postbacks, and plus counterflows. (NERC Glossary of Terms)
Available Margin
The difference between available resources & net internal demand, expressed as a percent of available resources. This is the capacity available to cover random factors such as forced outages of generating equipment, demand forecast errors, weather extremes, & capacity service schedule slippages.
Available Transfer Capability (ATC)
A measure of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses. It is defined as Total Transfer Capability less Existing Transmission Commitments (including retail customer service), less a Capacity Benefit Margin, less a Transmission Reliability Margin, plus Postbacks, plus counterflows. (NERC Glossary of Terms)
Average Demand
The electric energy delivered over any interval of time as determined by dividing the total energy by the units of time in the interval.
Avoided Cost
The cost the utility would incur but for the existence of an independent generator or other energy service option. Avoided cost rates have been used as the power purchase price utilities offer independent suppliers.
Back EMF
When AC current flows through a conductor an alternating magnetic field is created. This alternating magnetic field induces a voltage in the conductor. The induced voltage is called the conductor’s back EMF.
Back-to-Back HVDC
An HVDC system in which AC is converted to DC & then immediately back to AC. The HVDC transmission path is very short, normally consisting of a short section (a few feet) of buswork.
Backup Power
Power provided by contract to a customer when that customer’s normal source of power is not available.
Balancing Area ACE Limit (BAAL)
A generation control standard in which the allowable ACE bounds vary as a function of the Interconnection frequency error. The BAAL Standard is part of BAL-001-2 & replaces CPS2.
Balancing Authority (BA)
The responsible entity that integrates resource plans ahead of time, maintains load-interchange-generation balance within a Balancing Authority Area, and supports Interconnection frequency in real time. (NERC Glossary of Terms)
Balancing Authority Area
The collection of generation, transmission, and loads within the metered boundaries of the Balancing Authority. The Balancing Authority maintains load-resource balance within this area. (NERC Glossary of Terms)
Bandwidth
The acceptable range of a quantity. For example, if it is desired to hold the voltage at a 345 kV bus between 355 & 360 kV then the bandwidth is 355 - 360 kV or 5 kV.
Banking
Energy delivered or received by a utility with the intent that it will be returned in kind (as energy) in the future.
Base Load
The minimum amount of electric power delivered or required over a given period at a constant rate. (NERC Glossary of Terms)
Base Load Capacity
Capacity used to serve an essentially constant level of customer demand. Base load generating units typically operate whenever they are available, & they generally have a capacity factor that is above 60%.
Base Load Generating Units
Electric generating units that produce energy at a constant rate, usually at a low cost relative to other generating units available to the system. Baseload units are used to meet some or all of a given region’s continuous energy demand on a seasonal or daily basis, including at minimum load levels, & tend to operate non-stop except for maintenance or forced outages.